The present invention relates to aircraft navigation, and more particularly to vertical navigation guidance equipment having several important advantages over heretofore existing designs.
Vertical guidance equipment for general aviation aircraft is currently available in several different forms. For example, ILS installations provide localizer (directional) and also glide slope (vertical) capabilities. This is available at many but not all general aviation airports, and moreover, not all aircraft are equipped with the rather expensive ILS equipment.
Where the airport and/or aircraft are not ILS equipped, some other form of vertical guidance equipment is particularly advantageous. One way to accomplish this is as an adjunct to so called area navigation (RNAV) systems. RNAV is an outgrowth of the availability of a network of radio navigation aids called VORTAC's. These combine the capabilities of standard VHF omni directional range equipment with the TACAN tactical air navigation system to provide signals which are utilized on board an aircraft to provide bearing and range information for the aircraft with respect to the VORTAC facility. For RNAV systems, the VORTAC range and bearing are used to define a series of arbitrary reference locations called waypoints. These have no radio facilities, and are located at an address defined by the range and bearing to a nearby VORTAC. Then, the RNAV system computes aircraft range and bearing to the waypoint based on the waypoint address and the aircraft position with respect to the VORTAC.
RNAV systems of the type referred to above, are the subject of assignee's Bean U.S. Pat. No. 3,750,942 and Abnett et al U.S. Pat. No. 3,796,867, the disclosures of which are incorporated herein by reference to the extent pertinent.
Aircraft to waypoint range data generated by an RNAV system may be used directly to generate vertical guidance information in the absence of an ILS facility. By defining a waypoint at the runway, and establishing a suitable descent angle, the desired altitude as a function of the range to the runway can be computed directly from the resulting right triangle. Equipment for the above purpose is available, but it is characterized by several important disadvantages. First, the heretofore available equipment provides up-down steering commands based on comparison of the desired altitude with the reading of an encoding altimiter, thus, useful information can be derived only after a suitable barometric correction, obtained from a ground station at or near the airport. More importantly, however, is the fact that the resulting command is totally qualitative beyond the range of the up-down command needle. Thus, for a substantial departure from the required altitude, the up-down needle is pegged at "down" and the pilot has no quantitative indication of the actual error. In ILS operation, the aircraft might reach the ILS "outer marker" at an excessive altitude, and will be unable to make a straight-in landing. The same is obviously true in the case of a non-instrument, i.e. visual approach, wherein an outer-marker may not even be available.
Another difficulty with heretofore available equipment, relates to the establishment of a minimum descent altitude (MDA), i.e. that altitude below which the aircraft may not descend without having the runway in sight. For most airports, and typical general aviation aircraft, a appropriate value for the MDA is about 400 feet above ground level. However, certain conditions, such as radio propagation effects resulting from the location of the VORTAC, the particular direction of approach to the airport, length of runway, etc., may individually and/or collectively dictate some other value for the minimum descent altitude. Prior equipment provides a warning to the pilot at the range corresponding to the typical 400 foot minimum descent altitude, but no provision exists for selecting another MDA value.
Another disadvantage of prior equipment relates to selection of the appropriate descent rate. The usual descent rate for general aviation aircraft is 3.degree., i.e. approximately 300 feet per nautical mile. Other descent rates may also be desirable, and indeed, the Federal Aviation Administration has considered the possibility of a two segment glide slope having a descent rate of 600 feet per nautical mile at the beginning of the descent, and a 300 feet per nautical mile slope for the terminal portion of the descent. No provision for a variable descent rate has heretofore been available, except for designs not appropriate for general aviation use because of cost, weight or size.